Sculpted Science: Turn Milk into Plastic!

Introduction
Have you ever heard that plastic can be made out of milk? If this sounds far-fetched, you may be surprised to learn that from the early 1900s until about 1945, milk was commonly used to make many different plastic ornaments. This included buttons, decorative buckles, beads and other jewelry, fountain pens, the backings for hand-held mirrors, and fancy comb and brush sets. Milk plastic (usually called casein plastic) was even used to make jewelry for Queen Mary of England! In this activity, you will make your own casein plastic out of hot milk and vinegar.

Background
Plastics are a group of materials that may look or feel different, but can all be molded into varied shapes. The similarities and differences between different plastic products come down to the molecules that comprise them. All plastics are composed of molecules that repeat themselves in a chain, called a polymer. Polymers can be chains of either one type of molecule or different ones, which are linked together in a regular pattern. Also, in a polymer, a single repeat of the pattern of molecules is called a monomer, which can consist of just one type of molecule or include several different kinds.

Milk contains many molecules of a protein called casein. Each casein molecule is a monomer and a chain of casein monomers is a polymer. The polymer can be scooped up and molded, which is why plastic made from milk is called casein plastic.

Preparation
• Heat one cup of milk in a pan or on a stove top until it is steaming hot. Alternatively, you can microwave the milk in a microwaveable container by warming it at 50 percent power for five minutes. It should be about the same temperature as milk you would use to make hot cocoa; heat longer if needed.

Procedure
• Add four teaspoons (tsp.) of white vinegar to a mug or other heat-resistant cup.
• Add the cup of hot milk to the mug. You should see the milk form white clumps that are called curds. Why do you think the milk forms curds when it is added to the vinegar? What do you think they are made of?
• Mix the mug slowly with a spoon for a few seconds. What happens when the milk and vinegar are mixed together? Why do you think this is?
• Stack four layers of paper towels on a hard surface that will not be damaged if it gets damp.
• Once the milk and vinegar mixture has cooled a bit, use a spoon to scoop out the curds. You can do this by tilting the spoon against the inside of the mug to let excess liquid drain out while retaining the curds in the spoon. Collect as many curds as you can in this way and put them on top of the paper towel stack.
• Fold the edges of the paper towel stack over the curds and press down on them to absorb excess liquid. Use extra paper towels if needed to soak up the remaining moisture.
• Knead all of the curds together into a ball, as if it were dough. What you have in your hands is casein plastic. How do the kneaded curds feel and look different from the original ones?
• If you want to use the casein plastic to make something, you can color, shape or mold it now (within an hour of making the plastic dough) and leave it to dry on paper towels for at least 48 hours. Once it has dried, the casein plastic will be hard.
• Tip: To shape the plastic, the dough must be thoroughly kneaded. Molds and cookie-cutters work well or, with more patience, the dough can be hand sculpted. Food coloring, glitter or other decorative bits can be added to the wet casein plastic dough, and dried casein plastic can be painted or colored with markers.
• Extra: How does the amount of vinegar used affect the yield of casein plastic? To find out, you can repeat this activity—but in addition to testing four tsp. of white vinegar with one cup of hot milk, try also testing one tsp., two tsp. or eight tsp. of the vinegar, each with one cup of hot milk. To collect the most curds and get a better idea of their casein plastic yield, instead of scooping the curds with a spoon, you can pour the vinegar and milk mixture through a piece of cotton cloth (such as an old T-shirt) secured with rubber bands on top of a cup.
• Extra: In addition to vinegar, there are a lot of other acids that we encounter in the kitchen all the time, such as lemon juice, orange juice, soda pop and tomato juice. Do some of these common acids work better than others for making casein plastic?
• Extra: You used hot milk in this activity that was not at a specific temperature, but using hotter or colder milk might affect the casein plastic reaction. Design an experiment to investigate this. How does the temperature of the milk affect how much casein plastic you can produce?

Observations and results
Were you able to see the curds when the hot milk and vinegar were mixed together, and could you scoop out the curds to make casein plastic?

After you add the hot milk to the vinegar, small, white chunks—or curds—should become visible in the mixture. This is because adding an acid (such as vinegar) to the milk changes the latter's pH (acidity) and makes the casein molecules unfold and reorganize into a long chain, thereby curdling the milk. You should be able to use a spoon to separate the curds from most of the liquid. Additional drying of the curds with the paper towels should make the curds ready to knead into a ball and use as casein plastic, which can be molded and decorated.